Anomalies in the overall shape of the eye can cause visual disorders. Hyperopia ("farsightedness") occurs when the front-to-back distance in the eyeball is too small. In such a case, parallel rays originating greater than 20 feet from the eye focus behind the retina. In contrast, when the front-to-back distance of eyeball is too large, myopia ("nearsightedness") occurs and the focus of parallel rays entering the eye occurs in front of the retina. Astigmatism is a condition which occurs when the parallel rays of light do not focus to a single point within the eye, but rather focus to a region due to the fact that the cornea is aspherical and refracts light in a different meridian at different distances. Some degree of astigmatism in an eye is normal, but where the astigmatism is too pronounced, it must often be corrected.
Hyperopia, myopia, and astigmatism are usually corrected by glasses or contact lenses. Surgical methods for the correction of such disorders are known. These methods include radial keratotomy (see, e.g., U.S. Pat. Nos. 4,815,463 and 4,688,570) and laser corneal ablation (see, e.g., U.S. Pat. No. 4,941,093).
Another method for correcting those disorders is through the implantation of polymeric rings in the eye's corneal stroma to change the curvature of the cornea. Previous work involving the implantation of polymethylmethacrylate (PMMA) rings, allograft corneal tissue and hydrogels is well documented. One of the ring devices involves a split ring design which is inserted into a channel previously dissected in the stromal layer of the cornea. The device uses a minimally invasive incision through which the channel for the implant is created and, finally, through which the implant is inserted.
U.S. Pat. No. 4,452,235, to Reynolds, describes a method and apparatus for corneal curvature adjustment. The method involves inserting one end of a split end adjusting ring into the cornea of the eye and moving the ring in a circular path until its ends meet. The ends are thereafter adjusted relative to each other until the shape of the eye has assumed a desired curvature whereupon the ends are fixedly attached to maintain the desired curvature of the cornea.
Additionally, U.S. patent application Ser. No. 07/820,422, by Davenport et al., entitled "Method for Corneal Curvature Variation" suggests the use of ICRs for the correction of astigmatism. That disclosure does not suggest the use of ICRs having the inventive shape to alleviate astigmatism in the eye.
None of the prior art disclosures suggest the use of an ICR having a non-uniform shape about their periphery.